0Welcome!COMP1511 18s1

Programming Fundamentals

1COMP1511 18s1 — Lecture 14 —

Pointers + Structs + mallocAndrew Bennett

<andrew.bennett@unsw.edu.au>

2 

Before we begin… 

introduce yourself to the person sitting next to you

 

how are they going with assignment 2?

3

Overviewafter this lecture, you should be able to…

make progress on assignment 2

have a better understanding of pointers: what pointers are

how to use pointers why we use pointers

have a better understanding of structs

have a better understanding of memory in C: dynamic memory allocation using malloc

the difference between

(note: you shouldn’t be able to do all of these immediately after watching this lecture. however, this lecture should (hopefully!) give you the foundations you need to develop these skills. remember: programming is like learning any other

language, it takes consistent and regular practice.)

4

AdminDon’t panic!

assignment 2 (if you haven’t started yet, start ASAP

deadline extended to Sunday 13th May

assignment 1 tutor marking/feedback in progress

week 8 weekly test due tomorrow don’t be scared!

don’t forget about help sessions! see course website for details

5

let’s talk about pointers

6

Pointers?before we talk about pointers, let’s take a step back…

7

Variablesthink all the way back to week 1….

int age = 16;

what does this actually mean?

8

Variables and Functions

int main(void) {

int age = 16;

int height = 185;

}

9

Variables and Functions and Arrays

#define SIZE 5

int main(void) {

int age = 16;

int array[SIZE];

foo(array);

}

void foo(int array[SIZE]) {

int num = 10;

array[0] = 100;

}

10

Variables and Functions and Arrays and Pointers

#define SIZE 5

int main(void) {

int age = 16;

int array[SIZE];

foo(array, &age);

}

void foo(int array[SIZE], int *age) {

int num = 10;

array[0] = 100;

*age = 21;

}

11

re-visiting: structs

12

Arraysarrays are a collection of many of the same type of variable

int array[10];

// ten boxes that can each hold 1 int

[ ][ ][ ][ ][ ][ ][ ][ ][ ][ ]

// ten boxes that can each hold 1 int

[0][1][2][3][4][5][6][7][8][9]

13

Structsstructs are a collection of many of different types of variables

struct student {

int zid;

char name[MAX_NAME_LEN];

int ass1_mark;

};

// one box that can hold an int

[5112345]

// MAX_NAME_LEN boxes that can hold a char

[A][n][d][r][e][w][\0][ ][ ]

// one box that can hold an int

[94.5]

14

Structsstructs are a collection of many of different types of variables

struct student {

int zid;

char name[MAX_NAME_LEN];

int ass1_mark;

};

struct student andrew;

andrew.zid = 5112345;

andrew.ass1_mark = 94.5;

strcpy(andrew.name, "Andrew");

// one box that can hold an int

[5112345]

// MAX_NAME_LEN boxes that can hold a char

[A][n][d][r][e][w][\0][ ][ ]

// one box that can hold an int

[94.5]

15

Arrays of Structs?

struct student {

int zid;

char name[MAX_NAME_LEN];

int ass1_mark;

};

struct student students[NUM_STUDENTS];

// fill out one student struct in the array of structs

students[0].zid = 5112345;

students[0].ass1_mark = 94.5;

strcpy(students[0].name, "Andrew");

// fill out another student struct in the array of structs

students[1].zid = 9100123;

students[2].ass1_mark = 64.2;

strcpy(students[3].name, "Andrew");

16

let’s play: Intensity

17

Intensityyour task: write a program to play the game Intensity

the Intensity referee manages the game shuffles cards

deals cards

asks players for moves

etc

all input is given over standard input (i.e. scanf)

all output is given over standard output (i.e. printf)

18

Stateless AIan important concept to understand: your AI is stateless

it comes to life for one single move reads the input

thinks about what to do prints out its decision

19

Intensity Refereethe Intensity Referee runs the game

1511 intensity_referee

you can run your AI against it:

1511 intensity_referee your_ai_code.c

you can play interactively:

1511 intensity_referee -i

20

Valid Cards To Play

21

revisiting: memory

22

Scope and Lifetimesthe variables inside a function only exist as long as the function does

once your function returns, the variables inside are “gone”

(this is why you can’t return an array from a function!)

23

Lifetimeswhat if we need something to “stick around” for longer?

two options: make it in a “parent” function dynamically allocate memory

24

Lifetimesmake it in a “parent” function

void foo(void) {

int array[SIZE];

bar(array);

printf("%d", array[0]);

}

void bar(int array[SIZE]) {

array[0] = 123;

}

25

Lifetimesdynamically allocate memory

void foo(void) {

int *array = bar();

printf("%d", array[0]);

}

int *bar(void) {

int *array = malloc(SIZE * sizeof(int));

array[0] = 123;

return array;

}